DEFORMATION BEHAVIOR AND FRACTOGRAPHIC FEATURES OF DUCTILE Cu 47

Transcription

1 Deformation Rev.Adv.Mater.Sci. behavior 18(2008) and fractographic features of ductile Cu 47 bulk metallic glass 527 DEFORMATION BEHAVIOR AND FRACTOGRAPHIC FEATURES OF DUCTILE Cu 47 BULK METALLIC GLASS C. Duhamel 1,2, J. Das 1,3, S. Pauly 1, K.S. Lee 1 and J. Eckert 1,3 1 FG Physikalische Metallkunde, FB 11 Material- und Geowissenschaften, Technische Universität Darmstadt, Petersenstraße 23, D Darmstadt, Germany 2 SIMAP-CNRS, Institut National Polytechnique de Grenoble, 1130, rue de la Piscine, BP 75, St Martin d Hères, France 3 Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden, Helmholtzstraße 20, D Dresden, Germany Received: March 29, 2008 Abstract. Cu 47 bulk metallic glass rods of 2 mm diameter (Φ) are prepared by arc-melting and in-situ suction casting. The compressive deformation behavior under quasi-static conditions of the as-cast amorphous alloy exhibits a high yield strength ( MPa) combined with significant plastic deformation (up to 5%). The fracture surfaces of the deformed samples consist of a mixture of vein-like patterns, river-like patterns and smooth regions. The area covered by veinlike patterns decreases when the ductility of the material increases. Protuberances, which interact strongly with the shear bands, are found at the surface of the deformed specimens. 1. INTRODUCTION Among the recent bulk metallic glasses (BMGs) that have been designed, the Cu-base alloys have attracted extensive interest. A series of simple binary Cu-Zr [1,2] and ternary Cu-Zr-Al [3-5] BMGs and BMG composites have been reported. Moreover, it has been shown that minor addition of Y [6] or Ag [7] to the ternary system can enhance the glass forming ability. At room temperature, BMGs generally exhibit a high yield strength but a limited ductility (ε p < 1%) [8]. However, the mechanical behavior of the Cu-Zr-base BMGs combines both high stress and enhanced ductility with plastic deformation that can reach 16% [9]. Ductility in compression has also been reported for Pt-base (ε p = 20%) [10], Zr-base (ε p = 4.5%) [11], Pd-base (ε p > 170%) [12] or Ti-base (ε p = 14.8%) [13] BMGs. Different hypotheses have been proposed to explain the enhanced ductility: (i) high Poisson s ratio [10], Corresponding author: C. Duhamel, cecilie.duhamel@ltpcm.inpg.fr 2008 Advanced Study Center Co. Ltd. (ii) nanocrystallization inside the shear bands during deformation [14], (iii) liquid phase separation [15], (iv) chemical short- or medium-range order [9]. In this paper, the mechanical properties of a Cu 47 BMG are reported. The deformation behavior is studied in compression under two different strain rates. SEM investigations of the fracture surfaces of the deformed samples are carried out and the observed features are correlated to the macroscopic ductility. 2. EXPERIMENTAL The Cu 47 alloy was designed by adding 6 at.% to the binary Cu 50 Zr 50 glass forming alloy composition. The master ingots were prepared by melting the pure elements (Zr, Cu, and Al) under a Tigettered Ar atmosphere. 2 mm diameter cylindrical rods were obtained from the master alloys by

2 528 C. Duhamel, J. Das, S. Pauly, K.S. Lee and J. Eckert Fig. 1. X-ray diffraction patterns of a 2 mm diameter rod of Cu 47. The cross-sectional microstructure of the 2 mm diameter rod, observed using back-scattered electron (BSE) mode, is shown in the inset. using an in-situ suction casting facility attached to the arc-melter. The microstructure of the as-prepared rods was investigated by X-ray diffraction (XRD) in transmission geometry in a STOE STADI P diffractometer. Cylindrical specimens of 2 mm in diameter and 4 mm in length were prepared from the as-cast rods and tested under compression at room temperature using an Instron 5569 electromechanical testing machine at two different strain rates, &ε = s -1 and &ε = s -1. The true stress σ and true strain ε are calculated from the engineering stress σ E and strain ε E according to the following equations: a σ = σ 1 ε f E E a f, E ε = ln 1 ε, where ε E has been corrected from the rigidity of the testing machine estimated by comparison of the Young s modulus measured from the compression test to the one measured by ultrasonic sound velocity measurements. The fracture surfaces and the surfaces of the deformed specimens were investigated using a Zeiss DSM 962 scanning electron microscope (SEM) operated at 25 kv. 3. RESULTS AND DISCUSSION The X-ray diffraction (XRD) pattern of the as-cast Cu 47 alloy reveals two broad maxima characteristic for an amorphous structure (Fig. 1). The cross-sectional microstructure is shown in the inset of Fig. 1. No contrast is detected on the backscattered electron (BSE) micrograph. These results are in agreement with the bulk glass formation in (Cu 50 Zr 50 ) 100-x Al x (x = 0 to 10) cast rods of 2-5 mm diameter reported in the literature [3]. However, the presence of nanocrystals (d < 10 nm), non detectable by XRD and SEM and observed in TEM for Cu alloys [9], has not been checked. Fig. 2 shows the true stress strain curves at room temperature, under compressive loading at two different strain rates, &ε= s -1 (specimens A and B) and &ε = s -1 (specimens C and D). For all the samples, a high yield strength ( MPa) combined with a substantial ductility

3 Deformation behavior and fractographic features of ductile Cu 47 bulk metallic glass 529 Fig. 2. True stress true strain compression curves of the as-cast Cu 47 BMG loaded at a strain rate of s -1 (A and B) and s -1 (C and D). The intersections between the curves an the dashed line show the yield point at 0.2% deformation. The inset is a magnified view of the curve of sample D showing the work-hardening behavior and the serrated flow occurring during deformation. (ε p up to 5%) is observed. The inset in Fig. 2 reveals that, after yielding, (i) a further increase of the flow stress with deformation is registered and (ii) serrated flow occurs. The magnitude of the stress drops increases with deformation from 7-10 MPa at the first stage of the plastic domain up to MPa just before failure. The work-hardening behavior has already been reported for ductile Cu and Ti 45 Cu 40 Ni 7.5 Zr 5 Sn 2.5 BMG [9,13] whereas flow serrations have been observed for low strain rates [16]. Both the true stress strain curves and the data from Table 1 suggest a strain rate dependence of the deformation behavior. With increasing the strain rate from s -1 to s -1, the yield strength increases from MPa (specimens D and C respectively) to MPa (specimens B and A respectively) whereas a significant decrease of plastic deformation from % (specimens C and D respectively) to % (specimens A and B respectively) is shown in Table 1. The effect of strain rate on the deformation behavior of bulk metallic glasses at low temperature in compression is still unclear [16,17]. However, similar effect of the strain rate on the strength was observed in compression at room temperature in a Nd 60 Fe 20 Co 10 Al 10 BMG [18] and at 77K, in a Zr 57.4 Cu 17.9 Ni 13.4 Al 10.3 Nb 1 amorphous alloy [19]. In addition to the compression tests, the elastic properties of the Cu 47 alloy were determined from ultrasonic measurements. The Young s modulus (E), shear modulus (µ), bulk modulus (B) and Poisson s ratio (ν) are 86.5 GPa, 31.2 GPa, 131 GPa and These values are close to those reported for Cu 50 Zr 50 and Cu [9] alloys. The fracture surfaces of specimen A, deformed at s -1 and specimen C, deformed at s -1, are shown on Fig. 3. All specimens failed in a shear fracture mode but significant differences in the shape of the fracture planes and the features covering them are pointed out. They strongly depend on the macroscopic ductility. The fractured specimen A, which exhibits the lowest ductility, has a conical shape (Fig. 3a). The fracture surface con-

4 530 C. Duhamel, J. Das, S. Pauly, K.S. Lee and J. Eckert Table 1. Room temperature compression tests results under two different strain rates for four samples of Cu 47 : yield strength σ y, fracture stress σ f and plastic deformation ε p. Sample Strain rate (s -1 ) σ y (MPa) σ f (MPa) ε p (%) A B C D sists in homogeneously distributed well-developed vein-like patterns (Fig. 3b), which are the classical features for the fracture surface of BMGs deformed under compressive loading [20]. They are attributed to a high temperature increase in the shear bands which is also suggested by the few molten zones and liquid droplets appearing at the fracture surface (arrows in Fig. 3b). Contrary to specimen A, a mixture of three different features is revealed on the fracture surfaces of specimens B, C, and D as exemplified in Figs. 3c and 3d for specimen C. One should notice that these three samples exhibit a plastic deformation of, at least, 2.1%. As shown in Fig. 3c, for specimen C, one half of the surface is covered by well-developed vein-like patterns (zone α). Then, an intermediary smooth region appears (zone β). On the other half of the surface, close to the edge, a mixture of well-developed river-like patterns and smooth regions is found (zone γ). It has been observed that the area covered by the vein-like patterns is reduced with an increase in the plastic deformation of the sample. This is also consistent with the features observed on specimen A, which is the less ductile sample (ε p = 0.36%) and for which only vein-like patterns were observed. Similar mixture of vein-like patterns, river-like patterns and smooth regions has already been reported for Zr-base BMG composites and Cu 60 Zr 30 Ti 10 BMG deformed under compressive loading [21]. The deformed surface of specimen C shown in Fig. 3e exhibits multiple shear bands. Two main networks can be distinguished: primary shear bands (I), parallel to the fracture plane, and secondary shear bands (II), perpendicular to the first ones [9]. The spacing between the shear bands is around µm. The intersection of primary and secondary shear bands leads to shear offset (black arrows in Fig. 3e). Branching of the shear bands are also observed. The branched shear bands are wavy, as if they were regularly pinned as shown by the white arrows in Fig. 3e. Such a wavy propagation of the shear bands has been reported for Cu [22]. Fig. 3f shows a spherical protuberance coming out from the deformed surface of sample C. Similar features are observed for samples B and D but not on sample A. Such protuberances, with a diameter of µm, are either isolated and randomly distributed at the surface and/or agglomerated in one particular region (close to the fracture plane or at the opposite edge of the sample). They do not exist on the surface of the non-deformed rods and are supposed to form during deformation. However, their origin is still unclear. Profuse shear banding is observed close to these protuberances and the spacing l between shear bands is drastically decreased (λ < 5 mm). 4. SUMMARY The deformation behavior and the fracture surface morphologies of the Cu 47 BMG have been reported. The amorphous alloy exhibits high strength, enhanced ductility and work-hardening. The yield strength and plastic deformation are found to be strain rate sensitive. Examination of the fracture surfaces reveals different features, depending on the overall ductility of the deformed sample. When plastic deformation increases, the area covered by vein-like patterns decreases and new features such as river-like patterns or smooth regions appear. The surfaces of the deformed samples show that multiple shear banding occurs during deformation. Protuberances, with a µm diameter and that interact strongly with shear bands, are found at the surface of the deformed amorphous samples. Such protuberances have never been reported before. Further investigations need to be carried out in order to clarify their origin.

5 Deformation behavior and fractographic features of ductile Cu 47 bulk metallic glass 531 Fig. 3. Fractography of (a-b) sample A and (c-f) sample C: (a) overview of the fracture surface of the less ductile specimen (sample A); (b) enlargement of the vein-like patterns. The arrows indicate molten areas and liquid droplets; (c) overview of the fracture surface of a ductile specimen (sample C) showing three different features: vein-like patterns (α), smooth regions (β) and river-like patterns (γ), (d) enlargement of a region with the three features, (e) surface of the deformed sample C showing primary (I) and secondary (II) shear bands and shear bands with a wavy propagation (white arrows). Intersections of primary and secondary shear bands lead to shear offset (black arrows), (f) protuberance at the surface of the deformed sample C. ACKNOWLEDGEMENTS The authors thank M. Calin, S. Scudino, M. Stoica, S. Venkataraman, P. Yu, L.C. Zhang, W.Y. Zhang for stimulating discussions and U. Kunz, H. Lehmann, C. Wastmund for technical assistance.

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